Carbon dioxide indicator



United States Patent tion Company, Incorporated, New York, N.Y., acorporation of New York No Drawing. Application December 26, 1956 SerialNo. 630,473

9 Claims. or. 252-408) This invention relate to chemical indicatorsolutions responsive to the presence of carbon dioxide in respiratorygases.

There has been a great need' for a simple and inexpensive device for useinoxygen therapy and in anesthetic administration which is effective fordetecting the presence of carbon dioxide at relatively lowconcentrations. The need for such detection arises from the fact that itis necessary to remove the carbon: dioxide from a patients respiratorygases when these gases are confined and recycled to the patient.Conventional anesthetic administering apparatus and oxygen therapydevices employ an adsorbent charge such as soda lime which removes thecarbon dioxide content of these gases by contact. In time, however, theadsorbent becomes depleted and a breakthrough of carbon dioxide occursfollowing which the content thereof in the breathing circuit may rapidlyrise to an undesirable and possibly dangerous level. It is highlyadvantageous, therefore, that such detection devices be capableofresponse at relatively low concentrations of carbon dioxide.

Chemical indicators have been used for such purposes. Such indicatorsare in the form of a solution having a characteristic color so long asthe air stream or respiratory gases contain less than a' smallconcentration of carbon dioxide corresponding to the normal levelpresent in the breathing circuit. When the concentration reaches ahigher level, depending upon the sensitivity of the indicator, a colorchange occurs which thereby gives an indication of the existence ofhigher concentrations of the carbon dioxide. On such indicator solution,known as theDraper solution which has been used, comprises an aqueoussolution of calcium carbonate as a chemical buffer and from cresolpurple as a dye. This solution has been found fairly effective and hasbeen the most generally accepted carbon dioxide indicator used in themedical field. Generally, however, the known chemical indicators areeffective for reliably indicating the presence of carbon dioxide in abreathing circuit only above about 1 /2-%. Advances in techniquesrelative to the administration of anesthetics, particularly, now make itextremely desirable to permit the detection of carbon dioxide at levelsof Well below 1% and preferably of about /2%. However, none of the knownchemical indicators afford this degree of sensitivity. In addition, theindicators presently in use frequently provide poor contrasts in thecolor characteristics between normal and responsive conditions whichmakes it difficult to determine whether the carbon dioxide concentrationhas risen. Also many of the solutions are characteristicly unstableduring storage or in use' and must be replaced frequently in order toinsure reliable readings.

' It is, therefore, an object of the present invention to provide" achemical indicator solution for the detection of carbon dioxide inrespiratory gases which is responsive about /z% or less.

It is a further object of the present invention to provide such achemical indicator in which a sharply contrasting change in color occursin the responsive range of the indicator.

It is another object of this invention to provide an indicator solutionfor the detection of carbon dioxide in respiratory gases which producesa reversible, sharply defined color change when the carbon dioxidecontent reaches about /2 and which is relatively stable during storageand while in use.

In accordance with the present invention, a highly effective, sensitivecarbon dioxide indicator comprises an aqueous solution of an alkalimetal oxalate and a pH sensitive color dye. The most suitable andreadily available oxalate salt is sodium oxalate. The oxalate salt maybe used with various-indicator dyes by adjusting its concentration sothat the pH change of the solution corresponding to the change from acondition of equilibrium with air .to the condition of equilibrium withabout /2 CO falls within the pH transition interval of the dye.Generally, color indicators, which are responsive in the pH range of 6.6to 5.8, may be used advantageously and include, for example, brom cresolpurple, broin thymol blue, indo-oxine, p-nitro phenol, chlorphenol red,beta naphthol, brom thymol blue-brorn cresol purple, and brom cresolgreen-chlorphenol red. Molar concentrations of aboutt).0002 to 0.01 ofthe oxalate salt are eifective with these indicator dyes. Higherconcentrations of the oxalae salt tend to retard the responsiveness ofthe indicator with an undesirable resultant loss in sensitivity. Forthis reason, it is preferred to utilize the indicators which may beemployed with the oxalate salt in lower concentrations. Particularlyadvantageous results have been obtained, for example, by the use ofsodium oxalate with brom cresol purple as an indicator. With thissystem, molar concentrations of from 0.001 to 0.01 of the oxalate may beused substantially without significant loss of sensitivity, althoughoptimum resultsare obtained with about a 0.005 molar concentration. Withthis oxalatebrom cresol purple system its normal purple colorcorresponding to the atmospheric level of carbon dioxide concentration,undergoes an easily discernible change to a bluish green when the carbondioxide concentration increases to about 0.3%, and to a light green atabout 0.5%. -It thus affords an extremely sensitive means of detectionfor carbon dioxide concentrations well below those concentrations, whichit has been possible to detect heretofore by other chemical indicators.As the concentration of carbon dioxide increases substantially above/2%, the contrast in color of the solution becomes greater, so that atlevels of about 1%, where heretofore the change in color with otherindicators has been barely discernible, the change may be visiblydetermined with ease.

It is highly advantageous, in order to render the indicator solutiondescribed herein unaffected by prolonged storage or by exposure torespiratory gases during continuous use, to combine with it a smallamount of a nontoxic, non-ionic, water soluble anti-oxidationstabilizing agent. The stabilizer prevents the degradation of theindicator and maintains it in highly sensitive condition. A stabilizingagent, effective for this purpose, is glycerine. The concentration ofthe stabilizer does not appear to be critical. However, glycerine inconcentrations of about five percent by volume stabilize the solution tothe extent that it may be stored indefinitely and used continuously inconventional anesthetic circuits for many days without requiringreplenishing or replacement.

An effective indicator solution prepared in accordance with the presentinvention may be used by contacting therewith a sample of a gas to betested. This may be accomplished, for example, by a simple, bulbaspirator connected through suitable conduits to a breathing circuit,preferably at a point downstream of the carbon dioxide absorbent means.Such apparatus, for example, may be as shown in the Draper, UnitedStates Patent No. 2,136,226, in which the indicator solution is held ina transparent glass vial. By causing the gas sample to bubble throughthe indicator solution, a change from its normal color indicates thepresence of carbon dioxide above the minimum concentration at which thesolution is responsive. Thus, in the case of a sodium-oxalate-bromcresol purple solution of the concentration above described, themaintenance of the normal purple color of the solution will show thatthe carbon dioxide in the system is below 0.3%. Upon a change in colorof the solution to light green, for example, it will be known that theconcentration of the carbon dioxide in the system has increased to about0.5%. Such an occurrence will serve to give a timely indication to theoperator of the presence of such carbon dioxide concentrations, so thatsuitable precautions, such as the replacement of the absorbent charge,may be taken. As the concentration rises to about 1%, the indicator willturn to yellow and remain this color for concentrations over this value.When the concentration of the carbon dioxide in the system returns toits normal level, the indicator solution will revert to its originalcolor. The color variations will change, of course, for differentindicator dyes and each system must be calibrated accordingly todetermine the minimum concentration of carbon dioxide which produces avisible response. The brom cresol purple dye is especially suitablebecause of the contrast in color which occurs at the minimum responselevel.

Generally, the concentration of the indicator dye is not critical.However, the dye should be added in amounts within the limits of visualability to detect the changes in carbon dioxide concentration with thechanges in the color of the solution corresponding thereto. The optimumamount of dye corresponds to the amount giving a shade or depth of colorwhose change may be observed most readily.

It will be understood that the present invention is not intended to belimited to any particular theory or explanation of its mode ofoperation. However, it is believed that a greater understanding of theinvention may be had by a consideration of the equilibrium ionization ofcarbon dioxide in water, upon which the response of the indicatorsolutions depends, as expressed, by the following equation:

When carbon dioxide is present in a sample gas contacted with theaqueous solution, equilibrium of the above ionization is reached atwhich the solution has a corresponding pH value depending upon theconcentration of the carbon dioxide. It may be seen that the pH of thesolution will vary depending upon the concentration of the carbondioxide. The pH sensitive dyes are employed as a means of indicating thechange in pH of the solutions. The pH of the solution without theaddition of a buffer, however, is not a reliable basis for determiningthe carbon dioxide concentration, because the solution in this instanceis extremely sensitive to transient conditions and to minor, or trace,contaminants, such as minute amounts of soda lime that are frequentlyfound in the gases of an anesthetic circuit. The butter agents, such asthose heretofore used, donate additional HCO ions sufficient tostabilize the solution against these transient conditions. When providedwith small amounts of these buffers, however, the pH incrementscorresponding to the lower concentrations of carbon dioxide lie outsideof the pH transition intervals of the indicator dyes that would afford asuitable color contrast and a pH value capable of afiecting these dyesis reached only after a substantial increase in carbon dioxide hasoccurred. Thus as previously mentioned, the best detection heretoforepossible has been above about 1 /2%. The concentration of the bufferagent cannot be employed to adjust the pH of the solution, so that itwill more closely approximate the transition intervals of the usabledyes, because the concentration of the HCO; ions is then changed to alevel rendering the solution highly insensitive to changes in carbondioxide concentration. The indicator would then create no detectablesignal. The present invention, however, involves the unique discoverythat an alkali metal oxalate solution, as hereinabove described, affordschanges in the pH of the solution in the low range of carbon dioxideconcentrations of as little as /z% and less, which fall within the pHtransition intervals of the usable pH sensitive dyes. As a result, aclearly visible change in color is produced enabling detection of carbondioxide concentration of a much lower magnitude by means of a chemicalindicator. The exact manner in which the oxalate salt achieves thisresult is not clearly understood, although it is apparent that itaflords a mixed oxalatebicarbonate system in which the pH range isshifted so that a great sensitivity of the pH sensitive dyes is affordedat the lower carbon dioxide concentrations.

A preferred indicator solution, in accordance with the presentinvention, is prepared by dissolving from 0.6 to 0.7 gram (0.005 mol) ofpure reagent grade sodium oxalate in one liter of distilled water towhich is added 50 ml. of U.S.P. glycerine and 15 ml. of an 0.04% bromcresol purple dye solution. The dye may be prepared by dissolving 0.1gram of finely powdered brom cresol purple in 18.5 cc. of 0.01 N NaOHdiluted to 250 cc. volume with distilled water.

While the present solution has been described primarily as a visualindicator for carbon dioxide, it will be understod that it may be usedin connection with spectrophotometric instruments which utilize thechange in transmission characteristics of the solution. Thus, thesolution provided in accordance with this invention will give extremelyhigh spectrophotometric sensitivity over the range of 0.03 to 0.5%carbon dioxide in which a transmission difference of about 22.5% occurs.

The invention is not limited to the specific embodiment hereindescribed, but may be used in other ways with the departure from itsscope as defined in the following claims.

I claim:

1. An indicator for detecting the presence of carbon dioxide inrespiratory gases at concentrations above that in air comprising anaqueous solution of an alkali metal oxalate and a pH psensitive coloreddye responsive to produce a discernible color change in the pH range ofabout 6.6 to 5.8, said solution being characterized by undergoing adiscernible change in color when the carbon dioxide concentrationincreases to the level of response of said indicator.

2. An indicator for detecting carbon dioxide in respiratory gasescomprising an aqueous solution of an alkali metal oxalate having a molarconcentration between 0.0002 to 0.01 and a pH sensitive dye having a pHtransition interval responsive to produce a discernible color change inthe pH range of 6.6 to 5.8.

3. An indicator for detecting carbon dioxide in respiratory gasescomprising an aqueous solution of sodium oxalate having a molarconcentration of between 0.0002 and 0.01 and a pH sensitive dye having apH transition interval responsive to produce a discernible color changein the pH range of 6.6 to 5.8.

4. An indicator for detecting carbon dioxide comprising an aqueoussolution of an alkali metal oxalate and brom cresol purple as a pHsensitive, color indicator dye.

5. An indicator for detecting carbon dioxide comprising an aqueoussolution of sodium oxalate and brom cresol purple as a pH sensitive,color indicator dye.

6. An indicator for detecting carbon dioxide comprising 0.001 to 0.01molar concentration of an alkali metal 5 6 oxalate in aqueous solutionand brorn cresol purple as 9. An indicator according to claim 8containing glycapH sensitive, color indicator dye. erme as a stabilizer.

7. A chemical indicator for carbon dioxide comprising an aqueoussolution of sodium oxalate having a molar concentration of about 0.005and brom cresol purple as a 5 References Cited in the file of thispatent UNITED STATES PATENTS PH sensitive 2,136,236 Draper Nov. 8, 19388. An indicator according to claim 7 wherein the equiv- 2,601,173 SeltzJune 171 1952 alent of about 15 ml. of an 0.04% solution of said brornOTHER REFERENCES cresol purple dye is employed for each liter of said 10Kolthoff et aL: Acid Base Indicators, The MacMfllan aqueous sodiumoxalate solution. 00., New York, 1937, page 265.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No,2,890,177 June 9, 1959 Tho'burn Kilmer It is hereby certified that errorappears in the printed specification of the above numbered patentrequiring correction and that the said Letters Patent should readascorrected below.

Column 1, line 1, for "relate" read m relates line 4% for "On such" readOne such line 48, for "from" read brom coluzim :2 lines 43 and .44, for"with. the" read he without line 5G, for "psensi tive" read sensitive oSigned and sealed this 23rd day of. February 19630 (SEAL) Attest: KARLH, AXLINE, ROBERT C. WATSON Commissioner of Patents Attesting Officer

1. AN INDICATOR FOR DETECTING THE PRESENCE OF CARBON DIOXIDE INREPIRATORY GASES AT CONCENTRATIONS ABOVE THAT IN AIR COMPRISING ANAQUEOUS SOLUTION OF AN ALKALI METAL OXALATE AND A PH PSENSISTIVE COLOREDDYE RESPONSIVE TO PRODUCE A DISCERNIBLE COLOR CHANGE IN THE PH RANGE OFABOUT 6.6 TO 5.8, SAID SOLUTION BEING CHARACTERIZED BY UNDERGOINH ADISCERNIBLE CHANGE IN COLOR WHEN THE CARBON DIOXIDE CONCENTRATIONINCREASES TO THE LEVEL OF RESPONSE OF SAID INDICATOR.